Alkali metal alkoxides are used as strong bases in the synthesis of numerous chemicals, for example in the preparation of active pharmaceutical or agrochemical ingredients. In addition, alkali metal alkoxides find use as catalysts in transesterification and amidation reactions.
Alkali metal alkoxides (MOR) are prepared by means of reactive distillation in a countercurrent distillation column from alkali metal hydroxides (MOH) and alcohols (ROH), the water of reaction formed being removed with the distillate according to the following reaction.MOH+ROH MOR+H2O
Such a process principle is described, for example, in U.S. Pat. No. 2,877,274, in which aqueous alkali metal hydroxide solution and gaseous methanol are conducted in countercurrent in a rectification column. In basically unchanged form, this process is described again in EP 1 242 345.
Similar processes in which, however, an azeotroping agent, for example benzene, is used additionally are described in GB 377,631 and U.S. Pat. No. 1,910,331. The azeotroping agent serves to enable a separation of water and a water-soluble alcohol. In the two patents, the condensate is subjected to a phase separation in order to remove the water of reaction.
For instance, DE 96 89 03 also describes a process for continuously preparing alkali metal alkoxides, in which the water-alcohol mixture removed at the top is condensed and then subjected to a phase separation. In this case, the aqueous phase is discarded and the alcoholic phase is returned to the column at the top together with the fresh alcohol. A similar process is described by EP 0 299 577, in which the water is removed in the condensate with the aid of a membrane.
In practice, all processes have the disadvantage that, at the feed point of the alkali metal hydroxide solution—i.e. at the point in the column profile at which the concentration of the alkali metal hydroxide used is the greatest—the risk exists that solid precipitation occurs and the process has to be interrupted.